Alkaline Zinc and Polarized Anodes
Q. We have recently installed an alkaline zinc plating line. Our anodes polarize when we run the line. What is causing this, and how can we prevent the problem from occurring? M.L.
A. Two things can cause this problem: 1) The anode current density is too high; 2) The caustic soda (sodium hydroxide) concentration is too low. The anode efficiency of alkaline zinc plating baths is much higher than that of the cathode efficiency.
If you are attempting to maintain a one to one anode to cathode ratio in your bath it’s more than likely that zinc metal content will increase. To counteract this problem, quite often some of the zinc anodes are removed. A result of this is that the current density on the remaining zinc anodes increases to such a high level that they end up polarizing. What you have to do is substitute plain steel anodes in order to restore the one-to-one ratio.
You mentioned in your e-mail that you have a rack line. In a rack alkaline zinc plating line anywhere from 70 to 85% of the anode area will be plain steel. Sodium hydroxide concentration should be maintained somewhere in the area of ten oz/gal for each ounce of zinc content.
One of the problems in this bath is that during non-operation of the bath zinc anodes will tend to dissolve, increasing zinc content of the bath. There are two ways of handling this problem. One is to remove the zinc anodes when the plating bath is not operated. The second method is to use a small reverse current when the plating bath is not being operated. This is done by placing some of the steel anodes on the cathode bar and hooking up a small rectifier in such a manner that a small amount of zinc is plated onto the anodes.
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